As the amount of available landfill space decreases, and the number of articles manufactured from synthetic polymers increases, there is a need for environmentally responsible methods for disposal of these polymer containing articles. A welcome alternative to landfill disposal is the recycling and reuse of the polymers.
One significant limitation to the use of recycled synthetic polymer material is the imparted color. Colorants such as pigments and dyes are commonly added to polymer compositions for practical and aesthetic reasons and there is often a significant color difference between virgin polymer and a surface dyed fiber or otherwise colored polymer as it exists in a potentially recyclable polymeric article. The presence of a colorant may also impair the processability of the polymer during conventional recycling processes.
For example, a dye on the surface of recycled polymeric fibers may render the polymer unsuitable for reuse if the dye, should it survive processing without itself degrading or causing the polymer to degrade, is not consistent with the color desired for the article produced from the recycled polymer. In a worst case scenario, an article otherwise suitable for recycling would be rejected completely and deposited in a landfill simply because its color is undesirable or inappropriate for a particular downstream use.
A process for removing colorants from polymeric articles is therefore desirable. While a single process will in all likelihood not be applicable to all polymers, significant quantities of potentially recyclable polymer can be culled from single commercial sources. For example, a substantial quantity of polyamide fiber from post consumer carpet is potentially available for recycle. Approximately 40% of the face fibers in post residential carpet waste in the United States is surface dyed Nylon 6, while another 40% is surface dyed Nylon 6,6.
U.S. Pat. No. 4,227,881, incorporated herein in its entirety by reference, discloses a process for stripping dyes from textile fabrics which involves heating an aqueous solution of an ammonium salt, a sulfite salt and an organic sulfonate to at least 60° C. and adding the dyed fabric to the heated solution while maintaining the temperature of the solution.
U.S. Pat. No. 5,989,296, incorporated herein in its entirety by reference, discloses a process for removing indigo dye from denim scrap by extracting the fabric with an organic solvent in which the indigo dye is soluble at elevated temperatures, such as 1,1,2-trichloroethane. The solvent is then cooled and extracted with an aqueous solution containing a reducing agent.
U.S. Pat. No. 4,783,193, incorporated herein in its entirety by reference, discloses a process for stripping color from synthetic polymer products by contacting the colored polymer with dispersions of alkyl halides and aqueous solutions of bleaching/oxidizing agents to which specified quantities of acids and surfactant/wetting agents are added. Among the drawbacks are the use of potentially hazardous halogens and organic halides and the special provisions required to prevent escape of vapors which could cause environmental harm.
In general, processes which use harsh stripping agents damage or destroy the colorant and generate a chemical waste stream that must be treated or disposed. Such harsh methods can also generate irremovable colorant fragments or harm the polymer which may limit the downstream recycle ability of the color-stripped material.
Processes which involve the simple solvent extraction of a dye from a polymeric substrate are conceptually simple, and are known to be viable at least on an analytical scale to determine dyes which may be present in, for example, a polymeric fiber composition. However, a useful industrial scale process for removing a colorant from a recyclable polymer must remove most if not all of the colorant from the polymer, and as noted before, it is expected that a single process or single extraction system will not be sufficient for the efficient removal of all colorants from all polymeric substrates.
U.S. Pat. No. 6,083,283, incorporated herein in its entirety by reference, discloses a process for removing color and extracting dyes from acrylics and polyamides by contacting them with a swelling agent under conditions such that the swelling agent interrupts the molecular forces within the polymer matrix and opens the polymer structure sufficiently to remove natural and synthetic colorants dispersed throughout a polymeric article.
Published US Pat. Appl. 20060070188, incorporated herein in its entirety by reference, discloses a process for dye removal from the surface of fibers by contacting the fibers with a non-aqueous ester stripping composition preferably containing at least one cyclic ester and optionally containing a surfactant, an alcohol, or both. This process claims the advantage of not having swelling agents penetrate throughout the polymer matrix which swelling agents would then have to be removed.
There remains a need for a process which can be relied upon to efficiently and completely remove colorants, that is dyes and pigments, from polymeric substrates and commercial products comprising these polymeric substrates to allow for the ready recycling of the polymers contained therein. The present invention provides a practical, cost-effective and environmentally friendly process for removing colorants from these polymeric substrates, such as polyamide articles, for example, dyed nylon fiber, making the polymer ready for recycling and reuse.